1. Field of the Invention
The present invention relates to a ceramic heater which is built into a gas sensor capable of detecting the concentration of a specific gas component contained in a target gas to be measured, for example, the exhaust gas which is emitted from the internal combustion engine mounted to motor vehicles. The present invention further relates to a gas sensor equipped with the above ceramic heater, and to a method of producing the above ceramic heater.
2. Description of the Related Art
FIG. 10 is the sectional diagram of the conventional gas sensor 8 equipped with the built-in ceramic heater 9 in its axial direction. As shown in FIG. 10, the gas sensor 8 is capable of detecting the concentration of the specific gas (such as the oxygen concentration) contained in the exhaust gas emitted from the internal combustion engine mounted to the motor vehicle. The gas sensor 8 has the built-in the ceramic heater 9 and the gas sensor element 80. The ceramic heater 9 heats the gas sensor element 80.
The ceramic heater 9 has the heater base member 910 made of ceramic. The heater base member 910 has the heater element (omitted from FIG. 10) therein.
FIG. 11 is the cross section of the contact area between the external electrode pad 912 and the output terminal in the conventional ceramic heater 9 built in the gas sensor 8 shown in FIG. 10. As shown in FIG. 11, the external electrode pad 912 is formed on an outer surface of the heater base member 910. The external electrode pad 912 is electrically connected to the heater element in the gas sensor 8.
The external electrode pad 912 is made of tungsten (W). The outer surface of the external electrode pad 912 is covered with the protective film 914 formed by a nickel plating film, for example. Japanese patent laid open publication No. 2005-158471 has disclosed a gas sensor having such a construction.
As shown in FIG. 11, the protective film 914 is electrically connected to the external lead 92 by the brazing member 915 made of Au—Cu alloy, for example. This structure allows the external electrode pad 912 to have thermal resistance properties and to resist oxidation.
As shown in FIG. 10, the gas sensor element 80 is of a cylindrical shape, in the bottom part of the diagram. The gas sensor 8 further has the housing in which the cylindrical gas sensor element 80 is disposed, and the ceramic heater 9 is disposed in the gas sensor element 80.
The gas sensor 8 has the target gas measurement chamber 82 and the atmosphere gas chamber 83. In actual use, the front end outer peripheral surface of the gas sensor element 80 is directly exposed to the target gas introduced into the target measurement gas chamber 82. The inner periphery surface of the gas sensor element 80 is exposed to atmosphere air introduced into the atmosphere chamber 83. The external electrode pad 912 is exposed to the atmosphere air in the atmosphere chamber 83.
The target gas measurement chamber 82 and the atmosphere chamber 83 are sealed with the seal member 84 placed between the gas sensor element 80 and the housing 81. This structure protects the atmosphere chamber 83 from entry of the exhaust gas.
However, because the exhaust gas temperature increases according to recently strict automobile emissions control, there is a possibility of applying thermal load to the seal member 82, and as a result, the air-tightness capability between the target gas measurement chamber 82 and the atmosphere chamber 83 decreases. This could allow entry of the exhaust gas into the atmosphere chamber 83, and as a result, there is a possibility that corrosive components (such as nitrogen oxide) contained in the exhaust gas reaches the protective film 914.
As described above, because the protective film 914 is made of a nickel (Ni) plating film and the like, nickel (Ni) easily reacts with such a corrosive component, nitric acid that is generated from nitrogen oxide contained in the exhaust gas. Accordingly, there is a possibility that the nickel (Ni) component in the protective film 914 is corroded by nitrogen oxide.
On the other hand, as shown in FIG. 11, there is another technique to cover the outer peripheral surface of the protective film 914 and the brazing member 915 with a chromium (Cr) plating film. However, thermal stress can cause cracks in the Cr plating film that is applied to the brazing member 915. Corrosive components enter cracks generated in the Cr plating film, and finally reach the protective film 914 that is made of nickel (Ni). This corrodes the protective film 914.
FIG. 12 shows a cross section of the contact area of another construction between the external electrode pad 912 and an output terminal 920 in the conventional ceramic heater shown in FIG. 10.
As shown in FIG. 12, Japanese patent laid open publication No. JP 2006-91009 has proposed a structure to connect the external lead to the external electrode pad 912 with the output terminal 920, not to fix them through a brazing member. The output terminal 920 is a part capable of supporting the base end part of the ceramic heater 9 in its thickness direction. In the ceramic heater having the construction shown in FIG. 12, the external electrode pad 912 is covered with the primary protective film 916 made of a Ni plating film and further covered with the secondary protective film 917 made of a gold (Au) plating film in order to keep its corrosion resistance.
However, one or more pin holes are often formed in the secondary protective film 917 made of the Au plating film. The presence of the pin holes in the secondary protective film 917 causes a possibility that the corrosive components enter and finally reach the primary protective film 916 through the pin holes. This often causes corrosion of the primary protective film 916 made of a nickel (Ni) film.
For this reason, there is strong demand of obtaining a ceramic heater equipped with an external electrode pad with superior anti-corrosion and thermal resistance.